Circuit boards, such as multilayer printed circuit boards (PCBs), are widely used in the electronics industry. PCBs typically comprise one or more layers having conductive traces etched onto them, the various layers separated by a dielectric material, with interconnections possible between layers, for example using through-holes or vias. Circuit board design and production is often subject to various, often conflicting, requirements, such as demand for products which are compact, operate at high-speeds, have low costs, and have high reliability. Meeting these requirements presents an ongoing challenge for the industry.
Transmission lines, such as microstrip or striplines, are often used to reliably route high-frequency signals, such as microwave signals, from one area of a PCB to another. Such transmission lines comprise a conductive strip or signal trace separated from one or more ground planes by one or more dielectric layers. Theoretically, signals in such transmission lines propagate as an electromagnetic wave existing at least partially in the one or more dielectric layers.
It is often desirable to make electrical contact with points along a transmission line, for example by using a spring-loaded probe pin to contact the signal trace of a microstrip transmission line on the outer layer of a PCB. Such probing may be required when testing the circuit board for quality control or other analysis purposes, for example. However, for small signal traces, it may be difficult to guide the probe pin with sufficient accuracy to contact the signal trace, which may result in circuit test failures or other problems.
One solution to the above-mentioned problem is to provide an enlarged conductive pad at one or more locations along the signal trace to facilitate ease of electrical contact. However, the impedance of the enlarged pad typically will not match the impedance of the rest of the transmission line to which it is connected. This impedance mismatch or discontinuity may affect signal propagation through the transmission line, resulting in partial signal reflection, power loss, signal degradation, and/or general distortion, which may impact circuit performance. This can be particularly problematic at high signal frequencies. These effects last the lifetime of the circuit, whereas the test pad may only be used once or twice during manufacture.
The impedance of a transmission line or associated feature can be altered by changing its size and shape. For example, capacitance to ground can be changed by altering the dimensions of the trace or feature, and by changing the distance from the trace or feature to ground. For example, a formula expressing a relationship between impedance, feature width and dielectric thickness of microstrip transmission lines is given in “Transmission-line properties of a strip on a dielectric sheet on a plane,” by Harold A. Wheeler in IEEE Tran. Microwave Theory Tech., vol. MTT-25, pp. 631-647, August 1977. One property of this formula is that increasing the width of a trace or feature results in a decrease in characterstic impedance. Another property is that increasing the distance of the trace or feature to the ground plane results in an increase in characteristic impedance. However, mere knowledge of the relationships between dimensions and impedance of a feature does not necessarily lead to appropriate recognition of, or solution to, the above-mentioned problems related to impedance discontinuity or mismatch.
In an application note entitled “Optimizing Impedance Discontinuity Caused by Surface Mount Pads for High-Speed Channel Designs,” published by Altera Corporation, May 2008, Serial No. AN-530-1.0, a plane cutout method is disclosed for reducing impedance discontinuities between a high-speed data channel and either DC blocking capacitor pads or surface-mount technology (SMT) pads of Subminiature A (SMA) coaxial connectors. It is proposed to compensate for an impedance discontinuity due to sudden widening of high-speed traces into capacitor or SMT pads by removing a portion of the ground reference plane directly beneath the pads. The appropriate portion of ground plane to remove is determined by simulation trial and error. In the area where the ground reference plane is removed, the signal is said to reference a ground plane further away. However, there is no discussion of the configuration or location of this further ground plane.
Therefore there is a need for circuit board elements and associated methods for matching impedance between a transmission line element and a conductive pad connected thereto that is not subject to one of more limitations of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.